The present invention relates in general to current balancing between parallel-connected battery strings in battery packs for electric vehicles, and, more specifically, to a self-adjusting circuit for pairwise balancing of the current from battery elements using a magnetic balance.
Electric drives for hybrid (i.e., gas-electric) vehicles and for fully electric vehicles require a DC power source. The source typically is comprised of a battery. Common battery types such as lithium ion (Li-Ion) use a large number of cells stacked together into a battery pack. The output voltage and/or current capability of the battery pack can be greatly increased over the voltage and current provided by a single cell. Stacking of the cells can be done in series, in parallel, or a combination of the two. For example, two or more strings of series-connected cells can be connected in parallel.
A parallel battery arrangement may be desirable because under certain conditions, battery elements (whether single cells or strings of series-connected cells) that are connected in parallel can deliver high current levels with less overall Joule heating (i.e., resistive losses) than when connected in series. Unlike a series connection which ensures that all cells carry the same magnitude of current, however, the current magnitude from different parallel battery elements can become unbalanced (i.e., unequal) due to differences between the DC internal resistances (DCIR) of the parallel elements. The battery cell or string of cells having a lower DCIR will carry a higher current which leads to resistive losses and an increase in parasitic chemical reactions within the battery cell(s). The charge capacity and ability to deliver power may be significantly degraded in the corresponding cells, adversely affecting battery life.
For the foregoing reasons, it has been important to balance the current (i.e., equalize the current magnitudes) between different parallel branches. But such balancing has been difficult because the DCIR values can vary with time and with exposure to different environmental conditions such as temperature. Complicated, expensive electronic circuits and controllers have been required for performing the current balancing.